Reinforced soil and method

ABSTRACT

Composite mixtures comprise soil and from about 0.1 to 5 percent by weight of additive discrete fiber materials. A related method for improving the engineering properties of soil includes the steps of adding from about 0.1 to 5 percent by weight of discrete fiber materials to soil and mixing the materials and soil together to form a blend.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 914,871,filed Oct. 3, 1986, now patent No. 4,790,691, formerly titled FIBERREINFORCED SOIL AND METHOD.

TECHNICAL FIELD

The present invention relates to soils having improved engineeringproperties and a method by which the improvements can be imparted. Insubstantially every aspect of civil engineering and architecture, therelative strength of the soil as a support for foundations must beconsidered. Such areas as the construction of buildings, bridges,drains, roadways and the like require that the soil not yield under theload while land fills, soil embankments, slope treatment and the likerequire that the soil be protected from erosion. The present inventionenhances the strength parameters of the soil, increases the resistanceof the soil to punching shear and reduces the compressibility of thesoil so that lesser amounts may be employed in many instances. Soils cannow be strengthened without the addition of other components that havebeen used heretofore with varying degrees of success.

BACKGROUND ART

A variety of materials have been blended with soils to enhance orimprove the properties thereof. In early highway construction, soil androcks were mixed to provide a more stable, free draining, betterperforming roadbed. Lime has routinely been added to clay and siltysoils to reduce their plasticities and to reduce their swellingpotential. Portland cement has been added to several types of soils,being mixed in place or in a batch plant for achieving an improvedhighway base material. More recently, woven synthetic materials havebeen placed in horizontal layers of soil in order to achieve steep,stable earth slopes.

Examples of the last technique, involving the use of so-calledgeotextiles, have been described in the patent literature. U.S. Pat. No.3,934,421, for instance, is directed toward a matting of continuousthermoplastic filaments that are bonded together at intersections. Whenplaced in loose soil, the matting provides increased vertical loadbearing capacity and resistance to lateral deformation.

U.S. Pat. No. 4,002,034 also discloses a matting, anchored to theground, for preventing erosion. The matting is a multi-layered compositeproviding an uppermost layer having the finest fibers and least porespaces and a ground side layer having the thickest fibers and greatestpore spaces.

U.S. Pat. No. 4,329,392 provides a layered matting designed to inhibitrearrangement of soil particles. The matting comprises melt-spunsynthetic polymer filaments with macrofibers forming a web, a filterlayer of finer fibers bonded thereto and an intermediate layer of otherfibers therebetween. The mat has use below water level to controlerosion.

U.S. Pat. No. 4,421,439 is directed toward woven fabric, comprisingfilaments such as polyester, polyamides and polyolefins. The fabric ispositioned beneath sand, gravel, stones, clay, loam and the like at adepth of at least 10 cm. The invention is based on the particularconstruction of the fabric which gives it improved load bearingperformance.

Another unique configuration of geotextile material is disclosed in U.S.Pat. No. 4,472,086. The material is used as a reinforcement for theconstruction of roadways and on slopes and river banks to controlerosion.

Despite the wide-spread use of man-made or synthetic filaments infabrics, matting and the like as a reinforcement for soil, the foregoingpatents have not taught the use of individual fibers or other discretesynthetic textile materials blended with the soil. Discrete fibers havebeen employed heretofore in the reinforcement of concrete as set forthin U.S. Pat. No. 3,645,961. The patent discloses the use of nylon,polyvinyl chloride and simple polyolefins in lengths ranging betweenone-quarter to three inches (0.4 to 7.5 cm) to form a blast resistantconcrete.

Actually, polypropylene fibers have been used to modify the behavior ofconcrete for over 20 years. Improvement in water tightness, reduction incracks, toughness, ductility, and impact resistance have been noted.Steel fibers have also been used for this purpose with limited success.Nevertheless, few studies on fiber reinforced soil have been reported.Those that exist have generally centered around attempts to understandthe effects of roots of vegetation on embankment slope stability,particularly of earth dams. Thus, improving the engineering propertiesof soil in this manner appears not to have been investigated heretofore.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide acomposite soil mixture having improved load bearing capabilities andrelated engineering properties, thereby benefiting foundations andcolumn supports.

It is a further object of the present invention to provide a method forimproving the engineering properties of soils by the addition of variousdiscrete fiber or slit film fiber materials thereto.

These and other objects, together with the advantages thereof over knownsoil mixtures and methods of treatment, which shall become apparent fromthe specification which follows, are accomplished by the invention ashereinafter described and claimed.

In general, the composite mixture of the present invention comprisessoil and from about 0.1 to 5% by weight of discrete fiber materialsselected from the group consisting of man-made fiber forming substancesand fiberglass, mixed therein.

The method of the present invention includes the steps of adding fromabout0.1 to 5% by weight of discrete fiber or slit film fiber materialsselectedfrom the group consisting of man-made fiber forming substancesand fiberglass to soil and mixing the fiber and soil together to form ablend.

As used in the specification, the term "discrete" is intended to mean afibrous material that is individually distinct or one which is notmathematically continuous. These materials are further intended to besubstantially non-continuous or capable of being made non-continuous. Asused in the specification, the term "man-made fiber forming substances"isintended to embrace both cellulosic and non-cellulosic or syntheticbase materials.

It has been found that the present invention provides improvements of upto50% in engineering properties particularly in the punching shearcapabilities of certain types of soils by the addition of 0.5% discretefiber materials by weight to the soils. Improvements of up to 250% havealso been observed by the addition of 1.5% discrete fiber materials byweight to the soils. Resistance to punching shear is measured by theCalifornia Bearing Ratio or CBR test. Improved punching shear resistanceprovides for reduced pavement component thickness plus greater pavementlongevity which, in turn, are important considerations in theconstructionof roadways and parking lots.

Among the other engineering properties noted hereinabove are the averagetotal angle of internal friction, φ, average total cohesion, C, andaverage initial tangent modulus, E_(T), all of which significantly areimproved by the invention. Steeper side slopes for embankments arepossible inasmuch as the average angle of internal friction is improvedsignificantly by the addition of the discrete fiber materials disclosedherein. As a result, less fill dirt is necessary and transportationcosts can be reduced. Moreover, because space is often at a premium inhighway and embankment construction, by using soil having improvedproperties, lateral spacing can be reduced.

Soil reinforced according to the present invention also provides theability to reduce volume change or settlement in high fills because oftheimproved modulus. Likewise, the long term strength of backfill soilsbehindwalls, retaining structures and the like are improved sincegreater cohesion and angle of internal friction values, or shearstrength, producelower earth pressures thereby reducing the potentialfor lateral movement. Also, less structural support is required forsoils placed behind retaining structures. Finally, stabilizing the faceof fill slopes, whether they be landfill slopes or dredge spoil(underwater) slopes, is accomplished by this invention based upon theextremely favorable enhancement of soil strength and deflectioncharacteristics.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

As noted hereinabove, practice of the present invention is based uponthe addition of various discrete fiber materials into soil. As is known,the basic types of mineral soils are gravel, sand, silt and clay.Mixtures thereof give rise to coarse-grained soils, more than 50%retained on a No.200 sieve, and fine-grained soil, 50% or more passesthrough a No. 200 sieve. No attempt shall be made to discuss thevariations in soil types. Those skilled in the art are familiar with andcan refer to the Unified Soil Classification System published as ASTMStandard D2487. With reference thereto, soils with which the inventioncan be practiced includegravel, sand, silt and clay.

The conventional fiber and slit film fiber materials that are added tothe soil can be selected from the broad class of commercially availableman-made fiber forming substances as well as fiberglass. Generallyspeaking, the materials should neither affect the soil nor be affectedby the soil and therefore, the material should not mold, rot, mildew,dissolve or otherwise deteriorate in the soil environment but shouldmaintain its basic integrity throughout its useful life.

One exception would occur where only temporary reinforcement isrequired. In coastal areas, for instance, the occurrence of a rash ofsevere storms can erode the natural dune structure. Use of a degradablefiber could provide temporary reinforcement of the soil structure untilnature can again complete the task. Other instances can be envisionedwhere environmental considerations could be satisfied by employingbiodegradablematerials as opposed to the non-degrading types and thus,the present invention should not be limited to either type because inspecific situations one type may be clearly preferred over the other.

Preferred materials include the olefins, particularly polypropylene,polyesters, nylons, acrylics and glass, but should not be limited tothese. Degradable man-made fiber forming substances would include rayon,acetate, triacetate and biodegradable or degradable polyolefins.Practicalconsiderations include creep resistance, a strong trait ofpolyesters, and dispersibility of the fiber material in the soil,although the absence of either one of these properties should noteliminate a particular polymer. Typically, man-made fibers havingspecific gravities ranging from about 0.80 to 2.36 and fiberglass with aspecific gravity range of about 2.50 to2.70 are suitable.

The reinforcing materials can be divided into two categories, fibers andslit film fibers. Slit film fibers are described in greater detailbelow; however, it is notable that for practice of the present inventionthey arefiber-like, that is, their length to width or crosssectionaldimensions arecomparable to fibers. Hence, the term discrete fibermaterials has been employed herein to connote both fibers and slit filmfibers. These two differ primarily by configuration although both aresimilarly dimensioned and are employed in approximately the sameamounts. The soil can be reinforced with mixtures of both fibers andslit film fibers.

With respect first to the conventional fibers, configuration can beimportant, but is also not a controlling feature. Work reportedhereinbelow provided very favorable results with monofilaments. Yet,othercross-sectional configurations such as rectangular, square, round,oval, hollow and the like may further enhance soil cohesion or otherproperties.Additionally, embossed, multi-lobal, collated or bondedfibers, triangular,entangled multifilaments filaments or monofilamentsand fibrids and fibrilsare other practical types for soil reinforcementprovided they can be uniformly dispersed into the soil. The fiberconfiguration could also be slubbed, spiraled, gear crimped, saw-toothconfigured, gnarled, cork-screwed or otherwise deformed to developcohesion or other fiber/soilmatrix properties.

Fiber length can range from about 0.5 to 4 inches (1.25 to 10 cm) with0.75to 1.5 inches (1.9 to 3.8 cm) being preferred. Fiber diameter isbetween about 0.003 to 0.10 inches (0.076 to 2.5 mm) and is variabledepending upon the application. Fiber yield, i.e., denier, which is alength to weight ration, is between about 50 to 41,000. The amount offiber added tothe soil ranges from at least about 0.1 percent by weightup to about 5 percent by weight with 0.1 to 2 percent being preferred.Practically speaking, the upper limit is not dictated by operability butmore a matterof diminishing returns. Thus, for many fibers, once morethan about 2 percent have been added, higher performance values areoffset by economicsunless specific engineering properties, i.e.,increased shear strength, aresought. Nevertheless, amounts in excess of5 percent are not beyond the scope of this invention if such additionscan be justified.

In the case of fibrids or fibrils, length and cross-section dimensionsare variable and nonuniform. Fibrid and fibril lengths or bundle lengthsof from about 0.0394 to about 0.472 inches (1 to 12 mm) are preferred,with individual fiber diameters being subject to the manufacturingprocess. Generally, fibrids and fibrils will range from micro-deniers toabout 90 denier.

With respect to the slit film fiber materials, these are formed fromfilms and sheets of the foregoing man-made fiber forming substances thathave been slit into thin strips. These thin strips may be further splitor treated by conventional processes into fibrillated or roll embossedfilm constructions. The films and sheets can be cut with conventionalapparatusinto narrow strips having both pairs of opposed sides parallel,e.g., rectangles and parallelograms, two sides parallel, e.g.,trapezoids or no sides parallel, e.g., quadrangles and other polyganolstrips.

Thickness of these strips may range from 0.001 to 0.020 inches (0.025 to0.047) mm) and widths may vary as is necessary to achieve the finalweightof the product desired. Lengths of the strips are comparable tothat for the fiber materials disclosed hereinabove, namely from about0.5 to about 4.0 inches (1.25 to 10 cm).

Similarly, the amount of such strips added to the soil ranges from atleastabout 0.1 percent by weight up to about 5 percent by weight, justas for the fibers, with 0.1 to 2 percent being preferred. The strips canalso be deformed in various manners to develop greater cohesion and/orother properties with the soil.

In addition to the amounts of the fibers or slit film fiber materials ormixtures thereof that are to be added to the soil, another factor isdepthof the composite soil/fiber and/or film fiber mixture. For roadwayapplications, the composite should be about 12 to 24 inches (30 to 60cm) thick. For mass fills to supports for buildings, roads and all otherused,the composite should be graded into all fill materials.

Addition of the discrete fiber materials to the soil is usually at thesiteand can be facilitated by broadcasting or laying the fibers or slitfilm fibers or both and blending via blade, graders, discs or harrows orby mixing with pulverizing mobile mixers, hydrostatic travel mixers,shreddermixers and the like. It is to be understood that neither thecomposite nor the method of the present invention is to be limited byany technique of mixing inasmuch as these steps are well known to thoseskilled in the art.

In order to demonstrate the effectiveness of the present invention toimprove engineering properties of soil, several examples were preparedwith varying amounts of fiber and tested and compared against the samesoil without fibers as a control.

The soil used was taken from a site near Winnsboro, S.C. The sample wascollected from the surface, below the organic topsoil and vegetation.Thissoil has been derived from the in-place weathering of metamorphicrock found in the Piedmont physiographic province. This province ischaracterized by rolling hills of moderate relief that are generally the"foothills" to the Blue Ridge Mountains. This province extends fromAlabama through Maryland, including the states of Georgia, SouthCarolina,North Carolina and Virginia.

In order to classify the soils, tests were performed to determine thegrainsize, liquid limit, and plastic limit. Based upon these results,the soil was classified as a sandy silt, reddish brown in color. Averageindex properties include: liquid limit=52; plasticity index=15; specificgravity =2.79; and percent fines=88.5. According to the Unified SoilClassification System, the soil is classified as MH.

the fiber selected was a 30 mil (0.76 mm), crosssection polypropylene.The fibers were one inch (2.54 cm) long and exhibited an initial tangentmodulus of 820 ksi (5658 MPa). The relatively large fiber diameter andshort length were selected in anticipation of desirable applicationproperties, such as resistance to wind disturbance, bulking, curling andthe like. The particular fiber tested was a monofilament configurationwhich is typically characterized as having a round cross-section and iscylindrical in design.

The experimental work included soil classification tests as well asseveralquantitative tests to evaluate changes in engineering propertieswith increasing quantities of fiber added to the soil. The engineeringproperties of interest, including the testing method used to obtainthese properties, are summarized in Table 1. The classification testsand testing methods are summarized on Table 2.

                  TABLE 1                                                         ______________________________________                                        Test Methods Used to Determine Engineering Properties                         Engineering Symbol        Test                                                Property    & Units       Procedures                                          ______________________________________                                        Total Angle of                                                                            φ, degrees                                                                              ASTM.sup.a D-2850                                   Internal Friction         EM 1110-2-1906.sup.b                                Total Cohesion                                                                            C, thousands of                                                                             ASTM.sup.a D-2850,                                              pounds (kips) EM 1110-2-1906.sup.b                                            per square foot                                                   Initial Tangent                                                                           E.sub.T, kips per                                                                           ASTM.sup.a D-2850,                                  Modulus     square inch   EM 1110-2-1906.sup.b                                Resistance to                                                                             CBR, percent  ASTM.sup.a D-1883                                   Punching Shear                                                                ______________________________________                                         .sup.a American Society for Testing & Materials, volume 04.08, "Soil &        Rock; Building Stone" 1985                                                    .sup.b United States Army Corps of Engineers, EM 11102-1906 Laboratory        Soils Testing, 1970                                                      

                  TABLE 2                                                         ______________________________________                                        Test Methods Used to Determine Soil                                           Classifications and Other Basic Properties                                    Classification Symbol       Test                                              Property       & Units      Procedure                                         ______________________________________                                        Liquid Limit   LL, percent  ASTM D-4318                                       Plastic Limit  PL, percent  ASTM D-4318                                       Grain Size                                                                    Distribution   None         ASTM D-422                                        Specific Gravity                                                                             SG none      ASTM D-854                                        Standard Proctor                                                                             γ, pounds per                                                                        ASTM D-698                                        Maximum Dry Density                                                                          cubic foot                                                     Standard Proctor                                                                             w, percent   ASTM D-698                                        Optimum Moisture                                                              Content                                                                       ______________________________________                                    

The CBR test and its accompanying design curve is one of several methodsused to evaluate subgrades for flexible pavement design. It is used inmany areas of the world for designing highways, parking lots, andairfields, and is one of the most commonly used methods in the UnitedStates for pavement design.

Triaxial shear tests are used in geotechnical engineering practice toevaluate several relevant parameters. These include the angle ofinternal friction, the cohesion, the modulus of elasticity, peak shearstrength, and other parameters. Triaxial shear tests can be performedwith a varietyof consolidation and drainage conditions. Each of theseconditions providesdifferent insights into likely performance of thesoil under various loading conditions. For the work reportedhereinbelow, unconsolidated, undrained tests were performed n simulatedcompacted fill to evaluate any change in soil strength as it relates toan "end-of-construction" condition for foundations and embankmentslopes.

Each of the above tests was performed from three to five times in orderto achieve a qualitative degree of statistical confidence in theresults. No statistical evaluations were performed other thancalculating an average value from the test results. The results of thestandard Proctor compaction tests were used to identify densities andmoisture contents to which both CBR and triaxial shear test specimenswere prepared.

The liquid limit tests, plastic limit tests, specific gravity tests, andgrain size distribution tests were only performed on the naturallyoccurring soil. All other tests were performed on a control groupcontaining no fiber and soil that had 1/2%, 1% and 11/2% fiber, by dryweight, blended into the test specimens.

Test Procedures

Fibers were mixed with soil in the following percentages by dry weight:1/2, 1 and 11/2%. The fibers were blended by hand until they appeared tobe evenly distributed throughout the soil mass. Water was then added andblended into the soil by hand until thoroughly mixed. The moisturecontentof the soil/fiber blend was computed as the weight of waterdivided by the dry weight of the combined soil and fiber. Test specimenswere allowed to "cure" at least 24 hours after water was blended beforeperforming Proctor, CBR, and triaxial shear tests.

Three standard Proctor compaction tests were performed on the controlgroup(no fiber), and 5 standard Proctor compaction tests were performedon each group with fiber blended into the soil (1/2%, 1% and 11/2%). Themaximum dry density and optimum moisture content were then determinedgraphically,and the average values then computed for each test group.These average values of maximum dry density and optimum moisture contentwere then used as the basis for preparing the CBR and triaxial sheartest specimens.

Four sets of three CBR samples were prepared, including the controlgroup and each percent fiber. The specimens were molded in the CBR moldto approximately the average maximum dry density and optimum moisturecontentdetermined from the groups density testing (Proctor tests). Aftercompletion of preparation, they were soaked by immersion in a water bathfor four days, according to standard procedures, then tested. Testresultsare reported in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Summary of Proctor and California Bearing Ratio Test Data.sup.a                                            Ave. CBR      Ave. CBR                           Soil Description                                                                      Proctor Tests CBR at 0.1"                                                                          at 0.1"                                                                              CBR at 0.2"                                                                          at 0.2"                            __________________________________________________________________________    No Fiber                                                                              γ = 93.1 pcf                                                                   W = 29.1%                                                                            4.6           4.9                                       No Fiber                                                                              γ = 93.5 pcf                                                                   W = 28.9%                                                                            5.1           5.8                                       No Fiber                                                                              γ = 92.1 pcf                                                                   W = 29.3%                                                                            5.1           5.4                                       No Fiber                     4.9           5.4                                1/2% Fiber                                                                            γ = 94.5 pcf                                                                   W = 26.2%                                                                            10.8          11.3                                      1/2% Fiber                                                                            γ = 96.2 pcf                                                                   W = 26.3%                                                                            11.6          12.1                                      1/2% Fiber                                                                            γ = 94.0 pcf                                                                   W = 26.3%                                                                            11.6          11.8                                      1/2% Fiber                   11.3          11.7                               1% Fiber                                                                              γ = 96.9 pcf                                                                   W = 25.8%                                                                            13.0          13.3                                      1% Fiber                                                                              γ = 95.8 pcf                                                                   W = 25.7%                                                                            11.6          11.6                                      1% Fiber                                                                              γ  = 93.6 pcf                                                                  W = 25.9%                                                                            13.0          13.3                                      1% Fiber                     12.5          12.6                               11/2% Fiber                                                                           γ = 93.8 pcf                                                                   W = 26.8%                                                                            9.8           10.9                                      11/2% Fiber                                                                           γ = 94.7 pcf                                                                   W = 26.4%                                                                            10.7          11.2                                      11/2% Fiber                                                                           γ = 93.8 pcf                                                                   W = 26.7%                                                                            11.6          12.9                                      11/2% Fiber                  10.7          11.7                               __________________________________________________________________________     .sup.a Specimens remolded to approximately 100% of their standard Proctor     maximum dry density at approximately optimum moisture content.           

The data in Table 3 indicates that with the addition of only 1/2% fiber,there is more than a doubling of the CBR value over the control. Thiscould possibly lower the costs associated with constructing flexiblepavements. As in example, for a typical motel parking lot built over asoil such as the one tested, for a CBR value of 5 without fiber and with500,000 equivalent 18,000 pound axle loads applied to its ring roadsover a 20 year period, a typical pavement profile would consist of 3inches (7.6 cm) of Type I Asphaltic Concrete surface course with 10inches (25.4 cm) of crushed stone base and 10 inches (25.4 cm) of soilstabilized sub-base. With the fiber enhanced soil, the profile couldconsist of 1.5 inches (3.8 cm) of Type I Asphaltic Concrete over a 10inches (25.4 cm) crushed stone base and 10.5 inches (26.8 cm) ofstabilized sub-base. As the traffic on a highway, airfield, or parkingarea increases, more dramatic reductions in stone and asphaltthicknesses could result. This example serves to indicate the generalmagnitude of savings in conventional paving material, in this caseAsphaltic Concrete.

Four sets of three triaxial shear test specimens were prepared,including the control group and each percent fiber. The specimens weremolded to approximately 100% of the Proctor test groups average maximumdry density at its average optimum moisture content. Confining pressuresused were 1000, 3000, and 5000 psf (0.05, 0.14 and 0.25 MPa). The testresults are tabulated in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    Summary of Unconsolidated Undrained Triaxial Shear Test Data.sup.a                                      Ave. C  Ave. φ                                                                        Modulus.sup.c                                                                      Ave. Modulus.sup.c                 Soil Description                                                                      Proctor Tests C, ksf                                                                            ksf φ, Deg                                                                        Deg ksi  ksi                                __________________________________________________________________________    No Fiber                  1.80    18.5     596.4                              No Fiber                                                                              γ = 93.3 pcf                                                                   W = 28.7%                                                                            1.69    19.5    614.6                                   No Fiber                                                                              γ = 93.3 pcf                                                                   W = 28.9%                                                                            1.93    17.0    574.7                                   No Fiber                                                                              γ = 94.3 pcf                                                                   W = 27.9%                                                                            1.77    19.0    599.9                                   1/2% Fiber                2.33    21.3     638.8                              1/2% Fiber                                                                            γ = 95.5 pcf                                                                   W = 26.7%                                                                            2.40    21.5    658.5                                   1/2% Fiber                                                                            γ = 95.9 pcf                                                                   W = 26.5%                                                                            2.20    21.0    632.3                                   1/2% Fiber                                                                            γ = 95.9 pcf                                                                   W = 26.5%                                                                            2.40    21.5    6.25                                    1% Fiber                  2.39    19.8     584.5                              1% Fiber                                                                              γ =  95.2 pcf                                                                  W = 26.7%                                                                            2.23    18.5    569.6                                   1% Fiber                                                                              γ = 95.5 pcf                                                                   W = 26.5%                                                                            2.54    18.0    570.0                                   1% Fiber                                                                              γ = 95.6 pcf                                                                   W = 26.3%                                                                            2.39    23.0    614.0                                   11/2% Fiber               2.70    26.0     701.8                              11/2% Fiber                                                                           γ = 96.2 pcf.sup.b                                                             W = 25.4%                                                                            2.78    24.5    679.7                                   11/2% Fiber                                                                           γ = 96.3 pcf                                                                   W = 25.3%                                                                            2.57    27.0    723.8                                   11/2% Fiber                                                                           γ = 96.4 pcf                                                                   W = 25.1%                                                                            2.74    26.5    701.8                                   __________________________________________________________________________     .sup.a Specimens molded to approximately 100% of their maximum dry densit    and approximately optimum moisture content                                     .sup.b Average of 3 Test Specimens                                            .sup.c Initial Tangent Modulus at 3 ksf Confining Pressure               

The data reported in Table 4 show extremely favorable enhancement of thesoil strength and deflection characteristics. By graphing the data, alinear relationship between triaxial strength characteristics andincreasing fiber content is suggested.

As an example of how the presence of fibers in soil enhance onepractical application related to the triaxial shear test, a squarefooting 5×5feet (152 cm×152 cm) resting on the ground surface can beconsidered.First, where the soil has not been enhanced by the additionof fibers, the footing could theoretically support approximately 333,000pounds (151,182 Kg). This same soil with 1/2% fiber could theoreticallysupport about 500,000 pounds (227,000 Kg) (an increase of 55%) and with11/2% fibers, the footing could theoretically support about 833,000pounds (378,182 Kg);two and one half times as much as the nonreinforcedsoil.

Based upon the foregoing disclosure, it should now be apparent that thepresent invention carries out the objects set forth hereinbove. Itshould be apparent to those skilled in the art that the addition offibers and/orslit film fiber materials to a variety of soils is possiblejust as a wide variety of fibers and slit film fiber materials areavailable from which to choose. Although the invention has beenexemplified by the addition of a round polypropylene fiber to soil fromSouth Carolina, it is to be understood that such examples were providedto enable those skilled in theart to have representative examples bywhich to evaluate the invention and thus, these examples should not beconstrued as any limitation on the scope of the invention. Similarly,the length of reinforcing material, itsconfiguration and the amountadded to a given soil can all be determined from the disclosure providedherein.

From the test results reported, it should be apparent that the possiblebenefits of fiber reinforced soil are great as is also true for the useofslit film fiber materials alone or in conjunction with conventionalfibers.In addition to providing support for columns and foundations,reinforced soil as described herein can be employed to provide steeperside slopes for embankments and the ability to maintain stable fillslopes, including dredge spoil slopes. Other potential applicationsinclude reduction in thevertical and lateral movement of compatiblefill; use for erosion control in slopes; enhancement of long termstrength of backfill soils behind walls and, earth liners.

Thus, it is believed that any of the variables disclosed herein canreadilybe determined and controlled without departing from the spirit ofthe invention herein disclosed and described. Moreover, the scope of theinvention shall include all modifications and variations that fallwithin the scope of the attached claims.

I claim:
 1. Reinforced soil having improved engineering properties forconstruction and excavation purposes comprising:natural soil selectedfrom the group consisting of gravel, sand, silt, clay and mixturesthereof; and from about 0.1 to 5 percent by weight of discrete fibermaterials mixed therein to improve the punching shear resistance, thetotal angle of internal friction, the average total cohesion and theaverage initial tangent modulus of said soil, said fiber materials beingselected from the class consisting of man-made fiber forming substancesand fiberglass.
 2. Reinforced soil, as set forth in claim 1, whereinsaid soil is a low plasticity silt.
 3. Reinforced soil, as set forth inclaim 1, wherein said discrete fiber materials are polyolefinic. 4.Reinforced soil, as set forth in claim 3, wherein said discrete fibermaterials are polyolefinic.
 5. Reinforced soil, as set forth in claim 3,wherein said discrete fiber materials are selected from the groupconsisting of degradable or biodegradable polyolefins.
 6. Reinforcedsoil, as set forth in claim 5, wherein said soil is sand and saidbiodegradable polyolefin is utilized for beach stabilization. 7.Reinforced soil, as set forth in claim 1, wherein said discrete fibermaterials are polyester.
 8. Reinforced soil, as set forth in claim 1,wherein said discrete fiber materials are from about 1.25 to 10 cm longand have a thickness of about 0.12 to 2.5 mm.
 9. Reinforced soil, as setforth in claim 1, wherein said discrete fiber materials are from aboutone to 12 mm long and have a thickness of about 0.12 to 2.5 mm. 10.Reinforced soil, as set forth in claim 1, wherein the specific gravityof said man-made fiber forming substances ranges from about 0.80 to2.36.
 11. Reinforced soil, as set forth in claim 1, wherein specificgravity of said fiberglass ranges from about 2.5 to 2.7.
 12. Reinforcedsoil, as set forth in claim 1, wherein said discrete fiber materialscomprise conventional fibers and slit film fibers having a discretelength.
 13. A method for improving the engineering properties of naturalsoils for construction and excavation purposes comprising the stepsof:adding from about 0.1 to 5 percent by weight of discrete fibermaterials to natural soil to improve the punching shear resistance, thetotal angle of internal friction, the average total cohesion and theaverage initial tangent modulus of said soil, wherein said fibermaterials are selected from the class consisting of man-made fiberforming substances and fiberglass and said natural soil is selected formthe group consisting of gravel, sand, slit, clay and mixtures thereof;and mixing said materials and natural soil together to form a blend. 14.A method, as set forth in claim 13, using a low plasticity silt.
 15. Amethod, as set forth in claim 13, using polyolefinic discrete fibermaterials.
 16. A method, as set forth in claim 15, using polypropylenediscrete fiber materials.
 17. A method, as set forth in claim 13, usingdiscrete fiber materials selected from the group consisting ofdegradable or biodegradable polyolefins.
 18. A method, as set forth inclaim 17, using sand and said biodegradable polyolefin for beachstabilization.
 19. A method, as set forth in claim 13, using polyesterdiscrete fiber materials.
 20. A method, as set forth in claim 13, usingdiscrete fiber materials having a length of about 1.25 to 10 cm and athickness of about 0.12 to 2.5 mm.
 21. A method, as set forth in claim13, using discrete fiber materials having a length of about one to 12 mmand a thickness of about 0.12 to 2.5 mm.
 22. A method, as set forth inclaim 13, employing man-made fiber forming substances having a specificgravity ranging from about 0.80 to 2.36.
 23. A method, as set forth inclaim 13, employing fiberglass having a specific gravity ranging fromabout 2.5 to 2.7.
 24. A method, as set forth in claim 13, using discretefiber materials comprising fibers and slit film fibers.